As a catalyst to be used for hydrotreating hydrocarbon oil, for example, for hydrogenation, desulfurization, denitrogenation and decomposition of the same, a hydrotreating catalyst composed of a porous catalyst material such as alumina, silica, titania or active charcoal and active metals of a metal of the Group VI of the Periodic Table such as molybdenum or tungsten and a metal of the Group VIII of the same such as cobalt or nickel as supported on the material is used. The active metals are supported on the catalyst material in the form of their oxides which are not active. Therefore, suitable pre-sulfurization is applied to the catalyst so as to convert the oxides into the corresponding sulfides before use.
However, it is known that, in a hydrotreating catalyst, since the active sites of the catalyst are formed on the surfaces of the active metal oxides therein, the number of the active sites increases with enlargement of the exposed surface area of the active metal sulfides to give, as a result, a high-activity catalyst. For the purpose of enlarging the exposed surface area of the active metal sulfides in the catalyst, formation of a fine powder of active metal sulfides to give a fine dispersion of them has been attempted and various methods for the purpose have heretofore been proposed. For instance, EP0181035(A2) has disclosed a method of preparing a hydrotreating catalyst in which a mixed liquid comprising a nitrogen-containing organic compound such as nitrilotriacetic acid, ethylenedimainetetraacetic acid or diethylenetriamine and active metals is infiltrated into a material such as alumina or silica and finally the resulting material is dried at a temperature of 200.degree. C. or lower. In accordance with the method, complexing ions are formed between the active metal and the nitrogen-containing organic compound and the resulting complexes are supported on the material so as to prevent coagulation of the active metals and to attain high dispersion of them. As a result, the catalyst obtained by the method have a higher active value than conventional catalysts. Recently, however, a requirement of reducing the sulfur content in a light oil due to legal controls on the exhaust gas has been submitted (the report of making the sulfur content in a light oil to be 0.05% by weight or less, as submitted to the related government authorities). In order to attain the object as intended by the requirement, the catalyst as prepared by the method was found to be still unsatisfactory.
The present applicant already proposed a method of preparing a hydrotreating catalyst in which an aqueous solution of a mixture of a hydroxycarboxylic acid such as glycolic acid, tartaric acid, citric acid or gluconic acid and active metals is applied to a catalyst material by dipping or kneading and thereafter the resulting material is dried at a temperature of 200.degree. C. or lower. In accordance with the method, since a hydroxycarboxylic acid is used as a complexing agent and the catalyst is not finally fired but is merely dried at a temperature of 200.degree. C. or lower, the hydroxycarboxylic acid is not decomposed but is stably held on the catalyst material and the active metal ions are complexed with the stable hydroxycarboxylic acid so that the thermal coagulation of the active metal sulfides to often occur after presulfurization of the catalyst may noticeably be inhibited. As a result, the active metal sulfides may well be supported on the material in the form of a high dispersion. According to the method, therefore, a hydrotreating catalyst having a higher activity than other conventional hydrotreating catalysts to which a nitrogen containing organic compound has been added can be obtained characteristically.
Where high-level desulfurization, or so-called deep desulfurization, of hydrocarbon oil is attained by the use of a conventional hydrotreating catalyst, a much more severe operation condition for hydrotreatment than an ordinary operation condition for the same is needed. As a result, the catalyst used is easily deteriorated and the catalyst life is shortened so that the operation cost is naturally elevated. On the other hand, where the above-mentioned catalyst as obtained with a hydroxycarboxylic acid is used in deep desulfurization of hydrocarbon oil, the hydrotreating operation may well be carried out under the conventional operation condition so that the current operation cost level may be maintained in the case. However, it is desired to effect the hydrotreating operation at a lower cost for the purpose of making provision for unforeseen elevation of the cost of crude oil. Under the situation, if a hydrotreating catalyst having a higher activity could be developed, deep sulfurization of hydrocarbon oil could be carried out under a milder condition than the status quo. Therefore, it is an urgent necessity for those skilled in the art to develop such a high activity catalyst.